Unexpectedly, many chondrites, regardless of type, contain secondary minerals that formed as the primary mineral reacted with water. The reactions took place soon after the chondrules formed and often during accretion of the incorporating bodies (Doyle et al. 2015), sometimes even before accretion. In the case of the asteroid Ryugu, hydration was dated to within the first 1.8 Ma of solar system history (McCain et al. 2023). Water vapour molecules suspended in space were wetting the grain surfaces and making them stickier, accelerating the process by which the grains coalesced. Carbonaceous chondrites tend to be particularly water-rich, possibly because they formed beyond the ‘snow line’ within which the infant Sun’s heat would have prevented water from condensing.
Evidence for the wetness of interplanetary space doesn’t just come from asteroids and comets. All the terrestrial planets show signs of having been drenched by water – even Mercury. One of the most astonishing findings of the Messenger mission was that under the beshadowed walls of Mercury’s high-latitude craters water abounds, with deposits up to 20 m thick.
No less surprising is the evidence from Venus. It is so hostile to life that one might think it exists purposely to demonstrate that an Earth-like planet is not an inevitability. Although Venus today is 465° at the surface and shrouded under clouds of carbon dioxide and sulphuric acid, the high ratio of deuterium to hydrogen in its atmosphere implies that it once hosted a substantial ocean, subsequently evaporated or blasted away. Deuterium is an isotope of hydrogen, heavier than the ordinary form because the nucleus includes a neutron, and when combined with oxygen it produces a heavy form of water. The Sun’s ultraviolet radiation split the evaporated water into hydrogen, deuterium and oxygen, so that the lightest gas, hydrogen, and most of the deuterium escaped into space. Some of the deuterium remained in the atmosphere, while the oxygen oxidised the crust. Dissociation from atmospheric HCO would be another mechanism of hydrogen loss.
Until quite recently, the Moon was believed to be devoid of water, but in October 2009 the LCROSS mission discovered significant quantities after crashing part of the spacecraft into a crater close to the permanently shadowed south pole. Five months later, it was announced that millions of tons of ice lay hidden in craters around the north pole. However, because of the difficulty of understanding where it might have come from, scientists continued to doubt that water was present. Consequently it was still newsworthy when in 2018 a paper analysing data gathered by India’s Chandrayaan-1 probe gave the first definitive proof of surface ice. Some of the water may be a product of the solar wind, though this is not explain why the far side is much drier than the near side; that at the poles is undoubtedly ancient.
Significant amounts have even been found in fragments of rock from beneath the surface, for example, in olivine crystals that grew while the containing volcanic melt was as yet unerupted (Hauri et al. 2011) and in apatite crystals that formed 4.4 Ga ago when the crust was pounded by asteroids (Tartèse et al. 2014). Analysis of the younger basalts recovered by China’s Chang’e-5 mission suggests that by 2.0 Ga the interior was essentially dry (Hu et al. 2021).
Water covers most of the Earth’s surface, to an average depth of 4 km. In the nebula scenario, Earth should not have had oceans to start with, since it was too hot to retain whatever water might have been delivered by early-forming comets. The whole planet was an ocean of magma, as a result of heat released by radioactivity and by giant impacts, including the one that created the Moon. Yet water has been abundant on or in the Earth from as far back as datable minerals can take us, in geological time as early as 4.4 Ga ago. Where this water came from continues to perplex. At the beginning of the Archaean 4.0 Ga ago, geological evidence suggests that the entire planet was under water.
Mars’s early history is also puzzling. Today its surface is cold, but like the other rocky planets it has an igneous crust, so its surface once was molten. One meteorite from the surface of Mars shows the crust formed surprisingly early: ‘no later than 4547 Ma’ (Bouvier et al. 2018) or just 15 Ma after the formation of the planet itself, based on modelling. Not much more than 70 Ma after that, Mars was struck by asteroids. As the evidence comes from only one meteorite, we cannot categorically say that this was for the first time, but it is unlikely to have been an isolated occurrence. Mars’s surface is peppered with ancient craters.
Wherever one looks, Mars shows evidence of former water. The depression in its northern hemisphere once contained an ocean over 400 metres deep, covering a third of Mars’s surface. Dried-up deltas and valley networks fringe the basin. In other regions, splashes of sediment around impact craters suggest that the ground already had a mud-like consistency. Cycles of evaporation, re-precipitation and riverine runoff continued. As the water gradually seeped away, it reacted with iron in the rock, and the planet rusted, turning red.
Approximately 1 bar atmospheric pressure was needed to maintain liquid water on the surface, equal to Earth’s atmospheric pressure (Palumbo et al. 2018). High levels of CO2 built up as a result of volcanic outgassing, evidenced in the mineral FeCO3. Today the planet is colder, has only a 7 millibar atmosphere (nearly all CO2), and what remains of the water, liquid as well as frozen, is locked up beneath the surface and in large permanent ice caps around the poles.
Jupiter mostly consists of hydrogen and helium. The helium is best understood as a product of in-situ nuclear fusion, though the process is not now going on. At the equator, water makes up just 0.25% of the molecules in Jupiter’s atmosphere. The atmosphere also contains small amounts of oxygen, which combines with hydrogen to form H2O. At the equator, water makes up just 0.25% of the molecules in Jupiter’s atmosphere. As with the other giant planets, the atmosphere also contains substantial amounts of ammonia (NH3) and methane.
Io, one of the four moons that Galileo saw with his telescope, is the innermost and densest of the four moons. It is partly molten and has almost no water. Europa has a rocky interior with an icy shell, below which an ocean of salty water is inferred. Charged particles streaming from Jupiter are continually splitting the frozen H2O into oxygen and hydrogen. Ganymede, the largest moon, is 50% rock and 50% water-ice. Callisto’s low density suggests that it too is 50% water but, unlike the other three, it has no metallic core.
Saturn is best known for its rings, which mostly consist of ice particles. One of the rings is fed by vapour ejected from the moon Enceladus, which hides an ocean of water beneath its icy crust. Because the rings are undarkened by interplanetary dust, astronomers conclude that they must be relatively young. How they originated is not known. A collision between two icy moons is one possibility. Jupiter, Uranus and Neptune also have rings, but faint, unspectacular ones that evoke no wonder.
An ocean of water has also been discovered 20-30 km beneath the surface of Mimas, the smallest of Saturn’s regular moons. Although the surface is heavily cratered, the ocean is inferred to be remarkably young, less than 15 million years (Lainey et al 2024). Titan, the second largest moon in the solar system after Ganymede, has a water-ice crust with lakes and rivers of liquid methane and ethane (C2H6). Liquid water lies beneath its surface, beneath that probably a mantle of frozen water and a rocky core.
The internal composition of outermost planets is unknown. Uranus could be a mixture of water and rock, with the water fraction anything from 33 ± 11% to 70 ± 17% (Morf et al. 2024). Its innermost region could be 80% rock and the rest a mixture of hydrogen and helium, or there could be substantial amounts of carbon and nitrogen (Militzer 2024). The moons are predominantly rocky, but probably the five largest have oceans beneath their icy crusts, with dissolved ammonia and salts acting as antifreeze.
Because of its pale greenish-blue colour (not ultramarine as in some images) Neptune was named after the Roman god of the Ocean, and its moons after minor water deities. Its atmosphere, primarily hydrogen and helium, makes up about 10% of its bulk. Neptune’s colour comes from the absorption of red and infrared light by atmospheric methane. Though similar in size, Neptune is 30% denser than Uranus, so probably contains more rock. Little is known about the moons’ compositions, except for Triton.
the fragmentary nature of the TNOs – more than 100,000 objects over 50 km and trillions of objects 10–100 metres in size (Cooray 2006);- the low total mass of the Kuiper Belt objects (more easily determined than that of the Scattered Disc);
- the ‘surprisingly high level of dynamical excitation’ of the objects – highly elliptical orbits at a wide variety of inclinations;
- the largest objects appear to be rocky.
The largest Kuiper Belt Objects (KBOs) – Pluto, Haumea and Makemake – are around 70% rock, with a substantial ice component, and are classified as dwarf planets. Some of the smaller KBOs may be mainly ice. As with the asteroid belt, the vast number of such objects is thought to reflect collisions between larger bodies. The ‘dynamical excitation’ of objects formerly in Pluto’s vicinity is evident from the thousands of impact craters dotting its surface. The present state of the belt therefore does not reflect its primeval state, and its more recent history is one of disaggregation rather than aggregation.
The composition of KBOs large enough to be analysed is inferred from their reflected light. Interaction with polymer-producing cosmic rays and the Sun’s ultraviolet radiation has complicated their chemistry, but in simple terms the surfaces of the largest bodies are mainly water-ice, nitrogen and various carbon compounds. Smaller bodies are not cold or massive enough to retain significant amounts of volatile ices.
Several pre-scientific peoples believed that a celestial ocean existed above the terrestrial one. The Egyptians visualised the Sun travelling through the heaven in a boat. Babylon’s creation myth, Enuma Elish, related that how Tiamat, a personification of the primordial waters, was split into an upper ocean and a lower ocean. Similarly the Hebrews, without personification, maintained that the space containing the sun, moon and planets was created by dividing an initially single body of water.
Can ancient tradition and modern astronomical knowledge be reconciled? The Hebrew account suggests a protective, nebulous, circumambient shell not unlike the shape of the postulated Oort cloud. Interstellar space was warmer than today because of the radiation from the Milky Way’s once luminous nucleus. Over time, the Sun’s gravity caused much of the vapour – since it had no angular momentum – to migrate inwards. By 4.57 Ga ago in geological time, interplanetary space hosted a substantial volume of water. Water requires surfaces termed ‘cloud condensation nuclei’ to transition from vapour to liquid, the most plentiful source of which would have been dust particles from the explosion of the largest rocky planets. Dust would also have come from the extant rocky planets as they were struck by the debris. The Kuiper Belt and Scattered Disc would be the mixed remains of water from the circumambient shell and rock from the outermost planets.
Over time, the Sun’s gravity caused much of the vapour to diffuse inwards. By 4.57 Ga ago in geological time, interplanetary space may have hosted a substantial volume of water. Water requires surfaces termed ‘cloud condensation nuclei’ to make the transition from vapour to liquid, the most plentiful source of which would have been dust particles from the explosion of the largest rocky planets. Dust also would have come from the extant rocky planets when they were struck by the debris. Beyond Neptune, cooling and electrostatic sticking would have caused the droplets to consolidate into ice particles. The Kuiper Belt and Scattered Disc would be the mixed remains of water from the circumambient shell and rock from the outermost planets.
According to Genesis, the original Earth was destroyed in a cataclysm of waters. There were two agents of destruction. One was the springs of the great subterranean deep, which all at the same time exploded and flooded the planet. The other was the rain that for 40 days fell through ‘apertures of the heaven’. As a consequence of the rain, all life was blotted out.
At first sight, the rain is problematic. It is difficult to model an atmosphere that held that amount of water, and difficult to see how 40 days of rain, however torrential, could have drowned all mountains and resulted in all life’s being obliterated. Genesis implies that up to that point rain was unknown: Earth’s surface was watered from below, not above. While some water will have evaporated from lakes and inland seas and from the ground itself, rain was not a major component of the hydrological cycle. On the other hand, some water must have reached the Earth from interplanetary space and lingered in the upper atmosphere. But if so, what could have triggered its collapse?
Water might not have been all that rained on the planet. For example, in the story about Sodom and Gomorrah the text says that Yahweh rained sulphur and fire on the cities; Psalm 11 speaks of his coals raining down, i.e. volcanic bombs; the book of Joshua refers to a hail of ‘large stones’ or meteorites. The rain in Genesis might have been primarily a shower of asteroids. Vast amounts of dust kicked up by the impacts would have provided the nuclei for vapour in the upper atmosphere to precipitate as water, to say nothing of the water kicked up from the deep flooding the land. The Earth is known to have been struck by asteroids in its early history. The only question is whether the impacts were spread out over millions of years – a bombardment in slow motion – or concentrated in one short episode. Heavy bombardment would have obliterated terrestrial life.
